Arrangement for amplitude modulation



ARRANGEMENT FOR AMPLITUDEMODULATION Filed Aug. l2, 1936 CARR/EP ATTORNEYPatented Jan. 2, 1940 ARRANGEMENT Foa AMPLITUDE MODULATION Rudolf Urtel,Berlin, Germany, assignor to Telefunken Gesellschaftl fr DrahtloseTelegraphie m. b. H., Berlin, Germany, a corporation of Germanyifilmlication August 12,

. 1936, serial No. y95,519

In Germany August 12, 1935 12 Claims.

The invention relates to an arrangement for amplitude modulation inwhich the value of the sum of two alternating currents of equallfrequency is formed, and one or both partial currents are varied aboutan average value by one of several modulation voltages by means forinuencing the degree of amplification cfa controlled discharge tube.Such modulation ycircuits which may be designated as tube bridgecircuits, were found in the past to be extremely unstable in operation.Therefore in practice, such tube bridges for modulation purposes havenot found use, and other modulation arrangements have been resorted to.VThis invention overcomes the instability hitherto observed` in tubebridging circuits, which is due to the amplifier tubes proper, and whichapparently has its origin in the fluctuations of the emission of thetube cathodes. when measured on the mean emission amperage of thecathode.

depends, is comparatively much more iniiuenced by the said fluctuationsin the emission `than 'l the plate current, since the alternatingplate-currents are substantially compensated. On the `basis of thisexperience, it is proposed in bridge circuits used for modulationpurposes, that thel output voltages of the bridge be amplied, de-

tected, and modulation potentials filtered out,`

and to apply the direct potential obtained to the electrode of one orboth controlled kdischarge tubes of the modulation arrangement whichelectrode influences the amplification of the alternating currents.

Several embodiments of the invention `are shown in the drawing whereinFig. 1 shows a schematic circuit showing the means and method ofstabilizing tube bridges;

Fig. 2 shows a graphical analysis of the circuit operation forexplaining the invention;

Fig. 3 shows a modification, of one of the circuit elements of Fig. 1useful for television;

Fig. 4 shows a block diagram of a modification These fluctuations arehowever small But in tube bridges they pro- I duce a disturbingphenomenon because of the fact that the difference current of thebridgeon which the modulation arrangement principallyV lel resonancecircuit 8 tuned to the carrier frequency appliedto the primary windingof the transformer I2. Item Il' represents a high-frequency amplifierwhich amplies the entire band, and to which a detector` I9 is coupled bya transformer it. The detector circuit contains a yresistor 2B, acondenser 2l and afurther arrangement serving for filtering out themodulation potentials and consisting of a condenser 22 and a resistor23.' The point P is connected to the.

control grid I of the tetrode II in seriesv with a compensation voltagesource 24.

Thearrangement according toFig. 1 operates such that there will beapplied to the control grid I of the tetrode Ill the modulationfrequencies corresponding to the brilliancy of the picture point, andwhich may be derived for `instance from any picture scanning means.At'the two control grids 2 situated next to the anode, alterl hatingpotentials exist in opposite phase which.4

are taken from the two ends of the secondary winding of the transformerI2. Disregarding at .first the control action proposed in accordancewith the invention, and thus assuming that the 4grid I oi tube II to beat constant potential `relative to the cathode whereby in this case thegrid I is connected to the voltage divider connected to the battery I5,the arrangement operates in such way that owing to phase opposition ofthe potentials applied to the two grids- 2, l thealternating anodecurrents compensate each .other in the common plate circuit if the twogrids l have thesame potentials. If however, for instance at grid Ioftube Ill a higher positive potential appears than at grid I of tubeII, theny in the common anode circuit, the Aalternating anode currentsupplied by the tube I predominates, so that at the parallel resonancecircuit there appears a voltage having carrier frequency,

whose amplitude depends upon the potential dif?.r o

ference of the vtwo grids I.l The same action will .also take place forvariation in the emission of the cathode of the one of the two tubes,the characteristic of that tube being displaced. The remaining circuitrepresented in Figure 1 now' operates in such manner that suchfluctuations in the emission of the one tubeI is rendered ineffective bycorresponding potential displacement of the grid Ici the tube II.Itwill'be appreciated that the fluctuations in tube emission takeI placerelatively slowly compared with themodulation frequencies. In thehigh-frequency amplier I'I,-the voltage appearing at` the parallelresonance circuit will be amplified,` then detected by the arrangementI9 to 23 showing a rectifier and lter, and deprived of the modulationfrequencies by suitable choice of constants for the condensers 2| and 22and resistors 20 and 23. The potential appearing at point AP thus Variesin accordance with the variable emission of the two tubes. Thispotential is applied, through a compensation voltage source 24, to thegrid I of tube II, land hence influences the amplification of this tubesuch that the fluctuation of the input potential of the ampliiier IIproduced by variation in the emission will be compensated almostentirely. Under these conditions changes 'of the grid I potential in thetube I serves to produce a modulated carrier wave which is proportionalto the diierence between the anode current of the tubes I and II. Itwill be noted in this respect that when the bridge is balanced, thecarrier wave produces plate currents in phase opposition in the twotubes IIl and II, butinasmuch as the anodes of the two tubes areconnected together, the resultant current flowing through the .tunedcircuit 8 is zero since when the plate currentof the tube `Illincreases, the plate current of the tube II decreases in equal amount sothat the sum remains constant. When the grid I of the tube i0 hasapplied modulating potentials thereto, the balance is destroyed and ifthe modulating potentials are of positive polarity, the plate current ofthe tube IU increases so that the total increase of the plate current ofthe tube I0 is greater than the total change in an opposite sense of theplate current of tube II, and consequently, there will appear a Voltageacross the tuned circuit 8 which is proportional to the voltage appliedto the grid I of tube lil. This instantaneous change in Voltage,however, is not effective to rebalance tube II .since the filterconstants of the elements 20-23 are chosen to suppress high frequencycomponents.

The functioning of the circuit according to Fig. 1 can be explainedgraphically by means of the tube characteristics shown in Fig. 2. Inthis gure, there is shown in relation to the voltage ci at the grid I oftube Il, the input voltage e2 of the high-frequency amplifier il. Thebridge may be completely balanced at the value e1 at the grid of tubeI8. Therefore, the carrier-frequency potential at the parallel resonancecircuit is zero. For lower, or higher values of c1 the equilibrium ofthe bridge is disturbed, so that a carrier frequency appears at theparallel resonance circuit having the amplitude shown as ordinate of theline course a, b, c. At another emission current of the one of the twotubes other values of e1 are required to establish the equilibrium ofthe bridge, namely the values e1 and 81 respectively, andcorrespondingly the line course for 'the bridge is a, b, c, and a", b",0', respectively. In assuming that the Voltage at grid I of tube I iwould have the value El continuously,

at a Variation of the emission of the one tube, the carrier frequencyamplitude at the input of the amplifier Il would vary, i. e. the voltageez would Vary between the values e2 and 62', if however as proposed inaccordance with the invention, a potential is applied to the grid I oftube II, consisting of the compensation voltage ek and of the voltage esat point P, then only an unlike lower fluctuation of the voltage c2occurs. This can be readily seen when marking the voltage e2 at the*abscissa in Figure 2, and observing that the cotangent of anglecorresponds to the v ratio between the voltage e3 and the voltage e2.

Now the balance point of the bridge i. e. the

purpose of .the so-called auto-modulation.v

working point on the bridge line can be adjusted in a simple manner byvarying the value of the compensation voltage ek. Figure 2 shows that ata decrease of the value ek, the working point moves downwards on theright hand branch of the bridge characteristic, andy moves4 upwards withan increase-of ek.

When applying the described modulation arrangement to a televisiontransmitter, it will be desirable to apply to the grid i of tube I0 onlya modulation potential which corresponds to the alternating component ofthe picture brilliancy, and to Vary the compensation potential ek inaccordance with the average picture brilliancy. This can be accomplishedby inserting in place oi" the compensation voltage source 2li in Figurel, the circuit shown in Figure Herein, item. 25 designates a photocellon which the entire picture to be transmitted is projected, item 26 is`a D. C. Voltage source, and 2l is a resistor. Thus the photocell 2.5integrates the brilliancy values of the entire image, and the voltagedrop through resistor 2T thus corresponds to the average picturebrilliancy.

A bridge arrangement of the type described alsoaffords the transmissionof other signals in an especially simple manner than those correspondingto the picture brilliancy, such as for instance line and picturealternation signals. If the line and picture alternation pulses are tobe sentout by means of a shorter or longercomplete interruption of thecarrier wave, it is only necessary to take care that for thetransmission of completely black picture points, the bridge arrangement.is not modulated to the value @2:02-

and for sending out these special additional signals such assynchronizing pulses, it is only necessary to. disconnect from thebridge the high frequency feed.

This may be accomplished for instance by means of the circuit shown inFigure 4. Herein, the tube bridge isdesignated by 28, the carrier wavegenerator is represented by 29, and the keying. stage is designated by30, and may consist for-instance of a hexode which has the keyed.potential applied to one of its control grids.

Item` 3| is a generator for the synchronizing pulses, and 32, 33 are sawtooth generators of a cathode ray picture scanning means 34. Thecurrents furnished bythis generator are fed across an amplier 35 intothe grid i of tube I0 in the bridge in Figure 1. To simplify theillustration, the control arrangement for the continuous adjustment ofthe working point of the bridge is omitted in Figure 4.

The modulation arrangement proposed according to the invention may alsobe utilized for the By this is meant a modulation method in which forthe purpose of saving transmitter energy, the high frequency amplitudeis always adjusted to a value approximately proportionalV to theamplitude of the modulation voltage. Thus at a lower modulation voltage,the carrier frequency amp-litude is-small and vice versa. To this endthe compensation voltage and hence, the carrier frequency amplitude islto be rendered dependent upon the amplitude of the modulation voltage. Acorresponding arrangement is shown in Figure 5. Herein, justas in Figure4, item 28 representsA the bridge arrangement, and just as in Figure 1,item I1 is the high frequency amplier, and I9 to 23 represent thedetector arrangement.` In addition Figure 5 containsa further detector36 byl which the modulation voltagewill be detected,`

and whose output terminals are connected to a resistor 31 at which thecompensation voltage appears.

What I claim is:

1. In a balanced bridge modulator, the method of stabilizing the balanceor" the bridge against fortuitous unbalancing, which comprises producingenergy in accordance with fortuitous unbalancing of the bridge, andbalancing the bridge with the produced energy. l

2. In a balanced bridge modulator, the method of stabilizing the balanceof the bridge against fortuitous unbalancing, which comprises derivingfrom the bridge carrier Wave energy modulated by both signalling energyand spurious energy, deriving energy from the last mentioned energyrepresentative only of the spurious modulated energy, and balancing thebridge with the second mentioned derived energy.

3. In a balanced bridge modulator, the method of stabilizing the balanceof the bridge against fortuitous unbalancing, which comprises .derivingfrom the bridge carrier wave energy modulated by both signalling energyand spurious energy, rectifying the derived carrier Wave energy, lteringout a component of the rectified energy representative only of thespurious energy, and balancing the bridge in accordance with thefiltered component of energy.

4. A stabilized balanced bridge modulator comprising means for producingenergy in accordance with fortuitous unbalancing of the bridge, andmeans for balancing the bridge with the produced energy.

5. A stabilized balanced bridge modulator comprising means for derivingfrom the bridge carrier Wave energy modulated by both signalling energyand spurious energy, means for deriving energy from the last mentionedenergy representative only of the spurious modulated energy, and meansfor balancing the bridge with the second mentioned derived energy.

6. A stabilized balanced bridge modulator cornprising means for derivingfrom the bridge carrier wave energy modulated by both signalling energyand spurious energy, means for rectiiying the derived carrier waveenergy, means for lter ing out a component of the rectified energyrepresentative only of the spurious energy, and means for balancing thebridge in accordance with the filtered component of energy.

7. In a balanced bridge modulator, the method of stabilizing the balanceof the bridge against fortuitous unbalancing, which comprises derivingfrom the bridge carrier wave energy modulated by both signalling energyand spurious energy, rectifying the derived carrier Wave energy, lteringout a component of the rectified energy representative only of thespurious energy, supplying a source of constant energy, and feeding theltered component of energy in series with the supplied source ofconstant energy to the bridge to balance the bridge for the spuriousenergy.

8. A stabilized balanced bridge modulator comprising means for derivingfrom the bridge carrier Wave energy modulated by both signalling energyand spurious energy, means for rectifying the derived carrier Waveenergy, means for iiltering out a component of the rectified energyrepresentative only of the spurious energy, supplying a source ofconstant energy, and means for feeding. the filtered component of energyin series With the supplied source of constant energy to the bridge tobalance the bridge for the spurious energy.

9. In a balanced bridge modulator, the method of modulating carrier Waveenergy, which comprises the steps of adjusting the bridge to apredetermined balance, deriving energy from the bridge'representativeonly of spurious modulated energy, re-establishing the balance of thebridge in accordance with the derived energy, and photoelectricallyaltering the balance of the bridge in accordance With the integratedillumination of an optical representation to be transmitted.

10. A balanced bridge modulator comprising meanslfor adjusting thebridge to a predetermined balance, means for deriving energy from thebridge representative only of spurious modulated energy, means forre-establishing the balance of the bridge in accordance with the derivedenergy, and means for photoelectrically altering the balance of thebridge in accordance Withvthe integrated illumination of an opticalrepresentation to be transmitted.

11. The method of operating a balanced bridge modulator which comprisesderiving from the bridge carrier Wave energy modulated by bothsignalling energy and spurious energy, deriving energy from the lastmentioned energy representative only of the spurious modulated energy,balancing the bridge With the second mentioned derived energy, andunbalancing the bridge in accordance with the average value of thesignalling energy.

12. A balanced bridge modulator comprising means for deriving from thebridge carrier Wave energy modulated by both signalling energy andspurious energy, means for deriving energy from the last mentionedenergy representative only of the spurious modulated energy, means forbalancing the bridge with the second mentioned derived energy, and meansfor unbalancing the bridge in accordance with the average value of thesignalling energy.

RUDOLF URTEL.

